_form.haml

= form_for([:admin, @staff]) do |f|
  .row.mt-5
    .col-sm-12
      .form-group
        = f.label :specializations
      .form-group
        = f.select(:specialization_ids, options_from_collection_for_select(Specialization.all, 'id', 'title',  f.object.specialization_ids), {}, multiple: true, class: 'chosen-select form-control')

faculty.rb

class Faculty < ApplicationRecord
  has_many :faculty_specializations
  has_many :specializations, through: :faculty_specializations
  has_and_belongs_to_many :pages
  extend FriendlyId
  friendly_id :title, use: :slugged
  has_one_attached :profile

  def should_generate_new_friendly_id?
    title_changed?
  end
end

faculty_specialization.rb

class FacultySpecialization < ApplicationRecord
  belongs_to :faculty
  belongs_to :specialization
end

log

Started PATCH "/admin/faculties/holly-farmer-2" for ::1 at 2020-03-31 10:25:23 -0500
Processing by Admin::FacultiesController#update as HTML
  Parameters: {"authenticity_token"=>"lqKy+FsPrwFPYJzf6d9lNY/lxzNB7f8gnrMG/OGHUg6/+pDQ8Jv2SFI524VZbLUluFKnnrErrsbzbl9AtxegeA==", "faculty"=>{"title"=>"Ali Carpenter", "staff_title"=>"Business & Information Techno
logy Education Teaching Assistant", "email"=>"ali@pioneertech.edu", "phone"=>"580-418-4365", "website"=>"", "specialization_ids"=>[""]}, "commit"=>"Update Faculty", "id"=>"holly-farmer-2"}

Unpermitted parameter: :specialization_ids

specialization.rb

class Specialization < ApplicationRecord
  has_many :faculty_specializations
  has_many :faculties, through: :faculty_specializations
  extend FriendlyId
  friendly_id :title, use: :slugged
  validates_uniqueness_of :title
end